i didn't really feel the difference between the mercs exept for scout in term of speed so at least i know now there is a difference
also i don't see the use of engineer's railgun , like i feel like it does low damage and it's ammo is so expensive to make if you focus on making the intell before 6 am you may be able to buy ammo for it one time
i'd say for engineer's (red) issue make it so the last upgrade is a mini sentry on both halls that uses scrap every shot to damage the ennemy and make the railgun a rechargable weapon , like the pomson 6000 where you reload it at the middle area
and for the blue engineer i'd say make him like a battle engineer , where he can place a dispencer to heal himself and mini sentries that will attack you BUT he won't attack from the halls unless he has a sentry set up (also maybe making a place after the corridors so you can shoot the dispenser at the risk of the engi attacking you directly with increased damage)
that idea would make blue engineer a constant threat since he will be next to the halls 24/7 unless he dies or you waste ammo to break his stuff